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What is the difference between flow separation and turbulent flow?
For air flowing over a wing, both flow separation and turbulent flow involve disturbed flow next to the surface and smooth flow further away. At what point does one say "oh, this flow has changed from turbulence to separation"  (or vice versa) and why?
I am wondering if separation involves only a disturbed boundary layer, while turbulence can involve a wider disturbance such as in a stall?
For example, is it correct to say that in the stall, an already-turbulent flow (sometimes experienced as burbling) becomes detached?
Or that vortex generators, designed to re-energize a stagnating boundary layer, do so by creating turbulence in order to prevent separation?

No to the last question. Boundary layers can be turbulent.

Any wider turbulence would obviously include the boundary layer. But how is that related to flow separation?

Turbulent boundary layers delay separation. https://www.scientificamerican.com/article/how-do-dimples-in-golf-ba/

@GuyInchbald it's the boundary layer itself that is turbulent.

These comments and some answers have led me to expand the question with a couple of suggested examples.

Flow separation and turbulent transition are completely different phenomena.
Flow separation is driven by an adverse pressure gradient in the flow direction. On the top surface of a lifting surface, the flow has to decelerate and return to farfield pressure as it approaches the trailing edge of the surface. So there is an adverse pressure gradient near the back of foil topsides. The problem is that this pressure gradient penetrates the boundary layer right down to the skin of the foil, and the boundary layer has been slowed due to skin friction. The result is that getting the air outside the boundary layer slowed down to freestream velocity can result in the boundary layer flowing the wrong way, forwards over the wing. The flow has to go somewhere, so a bubble forms and the streamlines lift away from the skin. Laminar flow boundary layers are prone to this happening due to the velocity profile of laminar boundary layers.
An area with a strong adverse pressure gradient can also develop just behind the leading edge suction peak. This may form a bubble and flow often reattaches behind it. One common occurrence is for a laminar separation bubble to form and for turbulent flow to reattach behind it. These can be stubborn and tend to produce hysteresis in the lift vs AoA curve.
Separation is less likely to happen in turbulent flow, as it needs a greater adverse pressure gradient to happen.
Turbulence is strongly a function of freestream velocity, and only weakly a function of pressure gradients. Indeed, many turbulence models just use flat plate turbulence data (zero pressure gradient) and ignore the pressure gradients completely.
So the difference is that they are caused by different conditions. Separation needs an adverse pressure gradient strong enough to back up the boundary layer, and turbulence doesn't much care about the pressure gradient.

Creating turbulent flow on a smooth wing needs an adverse pressure gradient. Don't forget to mention that

@Abdullah I'm guessing you are referring to this bit - " All boundary layers start off as laminar. Many influences can act to destabilize a laminar boundary layer, causing it to transition to turbulent. Adverse pressure gradients, surface roughness, heat and acoustic energy all examples of destabilizing influences. Once the boundary layer transitions, the skin friction goes up. This is the primary result of a turbulent boundary layer. The old lift loss myth is just that—a myth." Adverse pressure gradient does have an weak effect on turbulent transition, but it is not required.

@Abdullah here is an example where the standard wall function of turbulent boundary layer, which does not consider pressure gradients, is upgraded to one that does. - https://www.afs.enea.it/project/neptunius/docs/fluent/html/th/node100.htm

@Abdullah You most certainly do not require an adverse pressure gradient to transition into turbulent flow.

That explains flow separation nicely, thank you. But it does not characterize turbulence - for example, might adverse flow not be seen as a simple form of turbulence? Why not?

@JZYL if we have a rough surface, and a very high Reynolds number, then yes, we can't get much laminar flow even with a favorable pressure gradient. I guess I didn't stress that enough. But laminar flow airfoils are designed to maximise the extent of the favorable pressure gradient, assuming of course that everything else is fine. Also, notice that boundary layer suction is a method to keep laminar flow from transitioning.

@PhilSweet I'm referring to A favorable pressure gradient is required to maintain laminar flow. Laminar flow airfoils are designed to have long favorable pressure gradients. It's right below your quote.

@PhilSweet I agree that turbulent boundary layers care much less about adverse pressure gradients. That's the reason why they delay stall, as you mentioned.

@Abdullah Not really. You don't need a "very rough surface" to trigger TS wave naturally. TS wave will be triggered on a smooth surface on a flat plate with zero incidence against the flow.

@JZYL But that still doesn't invalidate the fact that favorable pressure gradients are stabilizing for laminar flows, does it?

@Abdullah If you're talking about turbulence transition, then no, the shape parameter would usually continue to increase through the favorable pressure gradient. In any case, the local Reynolds number would continue to increase, so it would not be "stabilizing". If you're talking about how separation, that's a completely different topic than what I've taken issue with.

@JZYL so that quote is wrong?

@JZYL also, I most certainly am not talking about flow separation.

@JZYL Are you saying that by "maintain laminar flow" they mean "maintain attached laminar flow"?

@Adullah I'm not going to speculate on some random source. You can consult reputable books by Anderson or Drela on boundary layer, and they would indicate similar results on turbulence transition. An adverse pressure gradient will definitely promote laminar separation bubble at low moderate Reynolds number (as a source of natural transition), but it's not a necessary mechanism.

Guys – why don't you ask a new question? It's actually quite simple: An adverse pressure gradient slows speed in the main flow direction only, leaving crossflow speeds untouched. So these get higher relative to main flow speed which helps transition along. And regarding the "myth" of no lift loss from early turbulent transition: Just ask any owner of a glider with the Wortmann 67-170 airfoil and they can tell you it is anything but a myth. Explaining all this with sufficient depth won't fit here, though, therefore a new question would help.

On swept wings there is no need for the adverse gradient. The changing flow direction in the boundary layer is fully sufficient to trip transition. You should probably add that your answer is only valid for straight wings.

@PeterKampf No one mentioned anything about lift loss, or swept wing, here. This discussion is limited to whether "creating turbulent flow on a smooth wing needs an adverse pressure gradient." is a correct statement. No, one does not need an adverse pressure gradient to transition. That's all.

@JZYL Phil wrote in a comment "The old lift loss myth is just that—a myth." I addressed my comment to him and meant that false statement. No need to be upset.

At what point does one say "oh, this flow has changed from turbulence to separation"

At the point where the flow reverses direction.

Flow separation. The bold curve is the surface/wing.
Yes, that can happen.
Both turbulent and laminar flow can separate. Turbulent flow is in fact less likely to separate than laminar flow. This is why aircraft wings often have devices that deliberately create turbulence on the wing.
(Yes, separated flow produces negative skin friction, but at the price of huge pressure drag)
Here's a (badly hand-drawn) diagram showing the difference between laminar, turbulent and separated flows.

Just a clarification about stall. The stall is when the lift reduction caused by flow separation overwhelms the lift increase caused by flying at an increased angle of attack. Flow separation can happen without stall, and it will reduce the benefit gained from a higher angle of attack in proportion to the extent to which the flow is separated, but stall cannot happen without flow separation.
Indeed, many wings have separated flow at the trailing edge sometime before "stall" is reached. As one draws closer to "stall", the region of separated flow expands forward. The turbulence created by the wake of this separated flow hits the tail, causing "buffeting", which gives the pilot a warning that he is approaching stall. Airfoils that lack this feature, such as supercritical airfoils or sharp supersonic ones, tend to be dangerous to fly at slow speed with it's inherent high angles of attack.
And as you can see from the diagram, the flow separation at a given angle of attack is much worse for laminar flow than for turbulent flow. So the laminar separated case is more likely to be a stall than the turbulent separated case.

Lift vs angle of attack for thin, sharp wings vs thick ones. Airfoils designed for laminar flow fall in the thin category. And as above, just having or not having laminar flow on a wing can make a similar difference.
And yes, vortex generators prevent separation by creating turbulence, which causes high speed freestream air to get mixed with low-speed boundary layer, speeding up the boundary layer. It's a tradeoff between the drag of a turbulent boundary layer and the even greater drag and lift loss from flow separation.

But what is the technical distinction between separation and turbulence? Which is your diagram depicting and what would a diagram depicting the other one look like?

@GuyInchbald Sorry, It depicts separation. The bold line is the wing. The normal lines with rows of arrows show the velocities of the boundary layer.

Thank you. It makes sense now.

Would that last, detached turbulent flow be what is called the stall condition?

@GuyInchbald see above

Thanks. I can see that the laminar separated case is more likely to lead to a stall than the turbulent separated case, but if there is laminar separation prior to the stall, then surely the snap transition to stalled condition must be the sudden onset of turbulence? Would separation not grow smoothly? (BTW the DH.88 Comet racer is neither supersonic nor supercritical but has a notoriously sharp stall with no warning) I am also uncertain how the Kutta condition and reverse flow near the trailing edge relate, but that is going a bit off-topic.

@GuyInchbald apparently the DH.88 had thin wings. Also, see above again.

@GuyInchbald Separated flow may expand gradually or suddenly, depending on the type of wing. A wing that, as above, has sudden, sharp stalls will also have sudden, sharp increases in flow separation just before the stall point, since after all, flow separation is what leads to stall.

@GuyInchbald Think of a stall as a condition where pitching up decreases lift rather than increase it as would normally happen.

But wings can stall while still perfectly stable in pitch. So pitch instability cannot explain the sudden onset of the stall.

@GuyInchbald added a new diagram.

@GuyInchbald Buffeting doesn't necessarily mean you're unstable, it just means that you're being bumped around because your tail is being pounded by turbulence.

But what causes the sudden violent breakdown which characterises the stall of a thin wing? Is it a sudden change in separation or in turbulence or in something else? So far we have established only that it is not pitch instability.

@GuyInchbald: The wing is mildly unstable in pitch (more so with more camber) and only the wing-tail combination with an installed tail will be perfectly stable in pitch when the wing stalls. The sudden, sharp stall is caused by sudden flow separation right past the nose of the airfoil (causing separated flow over much of the airfoil) while the benign stall is caused by slowly increasing separation originating from the trailing edge and creeping forward with increasing angle of attack.

@PeterKämpf Thank you. The pitch instability bit is wholly irrelevant though, I wish you guys would stop bringing it up.

@GuyInchbald just FYI I was talking about the empennage being hit by the wing wake. And yes that's not that relevant. The whole point is that the thick airfoil has a gradually increasing level of flow separation as stall angle-of-attack is approached, which warns the pilot

Think of the boundary layer as a multi-lane highway with rubber cars which can bump into each other. This highway has a sticky curb on one side and the cars are a bit sticky themselves, so cars near that curb get the slower the nearer they are.
In one case the cars stay in their lanes and the rightmost lane, right next to the curb, (sorry, you Australians, Japanese or Indians: For you that would be the leftmost lane) is occupied by the slowest vehicles. Speed increases with each lane more distant from this slowest lane since cars rub along nicely. This is like laminar flow.
Now traffic changes and the drivers switch lanes frequently. The result is that cars in the slowest lanes have to speed up. New lanes join the fastest lane from time to time so the speed in the fastest lane will not slow down. Speed is now much more equal across lanes but the whole highway grows wider to accommodate all those new lanes with fast vehicles. This is like turbulent flow.
While in laminar flow the parcels of air all flow in the predominant flow direction, in turbulent flow there is a lot of crossflow, so those parcels get bumped along if friction with the wall (the sticky curb of the highway, to stay in the picture) slows them down too much. This needs a constant addition of new, high-energy parcels so the whole boundary layer is thicker and has a fuller speed profile.
However, if the speed gradient along the predominant flow direction is negative (say, in the recompression area in the rear upper half of an airfoil), the cars in the joining lanes become slower and the slower lanes slow down, too. It's as if they obey a sequence of speed limits that tell everyone to reduce their speed by some MPH. And then some more. If the speed near the curb (in the slowest lane) drops to zero and then reverses, flow separation has occurred. Now the slowest lane fills up with vehicles from both directions which pushes the cars in the adjoining lanes further out. The highway width explodes.
This can both happen with no or much lane changing; the result is the same. When it happens with no lane changing and drivers change their mind about that detail further downstream, the new cars joining will now bump all others along and get traffic moving again. This describes a laminar separation bubble with reattachment downstream.

I am wondering if separation involves only a disturbed boundary layer, while turbulence can involve a wider disturbance such as in a stall?

Every flow separates at the trailing edge. With too much angle of attack, this separation creeps forward on the upper side on thick airfoils or a new separation starts past the suction peak near the nose on thin airfoils. This separation, when extensive enough, causes loss of lift and defines the stall. Both laminar and boundary layers can experience this.
A special case is a laminar separation bubble which occurs past the suction peak but the subsequent transition to turbulent flow causes reattachment. This can still be followed by a separation of the turbulent boundary layer later on.

For example, is it correct to say that in the stall, an already-turbulent flow (sometimes experienced as burbling) becomes detached?

Yes, but also a laminar boundary layer can separate and cause stall (mostly at model airplane scales and smaller). The "burbling" you mention is not caused by this but by larger eddies hitting the tail. This indicates a major separation near the trailing edge on the inner wing but with no or little loss of lift. This kind of turbulence is different from that in a boundary layer and of a much larger scale.

Or that vortex generators, designed to re-energize a stagnating boundary layer, do so by creating turbulence in order to prevent separation?

Yes. Vortex generators add more high-speed lanes to the traffic in the boundary layer. They also help to fix the location of shocks in transsonic flight.

Great. Next time I get on a plane, I'm going to look at the wing and see it full of little rubber bumper cars bounding all over the place. :)

Now what has the UK done to you that it deserves no mention? :-)

I would really really avoid mentioning molecules at all. Turbulent and laminar flows are all  about the continuum. The molecules are completely chaotic in either. Individual molecules start to matter at completely different scales, the mean free path in air is around 70 nm. There is a good reason why fluid parcels or particles were invented https://en.wikipedia.org/wiki/Fluid_parcel

@VladimirF: Yes, that makes sense. I replaced them with "parcels of air".

@TooTea: They infected too many countries with the disease of driving on the wrong side of the road. But maybe I should give the south of Africa a honorary mention.

As one who understands the good nature of Yankee jibes in ineffectual defence of driving on the wrong side of the road, I would note that we Brits chose the left in the interests of road safety - to face the oncoming horses and pedestrians. Even today our Highway Code recommends that where there is no pavement (which some understand only as "sidewalks") pedestrians should face the oncoming traffic. More paranoid countries followed the horses in avoiding the oncoming rider's sword arm. But I only ever met rubber bumper cars at the fairground and none had a reverse gear.

@PeterKämpf I wish you would have a look at the comments section of Phil's answer

@Abdullah. Done.

authors: Vladimir F Героям слава


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What controllers are used in Airbus/Boeing fly-by-wire systems?
I was wondering, what controllers are used in airbus or Boeing aircraft fly-by-wire systems to control servos? Who manufactures such controllers, or are the systems controlled from flight computer directly?

The systems used by complex aircraft are typically made specifically for an aircraft platform and all the different manufacturers of both air-frames and the control systems will have slightly different system architectures.
For example, a system will have a flight control "computer" made by one manufacturer, this will send commands via a data bus to the actuator control unit which will move the control surface via a hydraulic actuator.
Another may have the flight control computer directly control the actuator.
Remember that with flight control computers there will be multiple computers all performing the same task, providing redundancy should one fail. Duplex and Triplex flight control systems have historically been used.
Companies like BAE Systems, Thales, Rockwell Collins, GE Aviation, Parker Aerospace, Safran are just some examples of the many companies involved in the complex supply chain for aircraft parts.
Flight control systems for a single aircraft may be made by separate manufacturers to ensure that no single design fault causes a catastrophic failure or loss of control. This is known as dissimilarity. One manufacturer might make a flight control computer with a PowerPC processor and another will use something totally different.
In other cases, the same ASIC (Application Specific Integrated Circuit) will be specified to two separate integrated circuit manufacturers who will both build the same part.
This interesting article covers some of the points covered above
https://www.aviationtoday.com/2001/10/01/a380-more-electric-aircraft/

source: Stack Exchange

license: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/4.0/

sort: votes


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What does it mean when an aircraft engine is de-paired?
What does it mean when an aircraft engine is de-paired? e.g. the PW1000g engines being de-paired because of issues in the LPT or the Rolls Royce Trent 1000 engines due to excessive corrosion on some of the turbine blades?

De-pairing means literally braking up pairs. In the case in question, it means that if an aircraft is equipped with engines of a certain specific type, one of the engines has to be replaced with a version that has been upgraded.
The reason for the de-pairing is to mitigate the risk of a dual engine failure: Having two engines that are both affected by the same issue considerably increases the risk of both engines failing on the same flight. At least RR Trent 1000 and P&W 1100G have been issued a de-pairing AD by EASA.
Article on RR Trent 1000 de-pairing AD on Airlineratings.com
Article on P&W 1100G de-pairing AD on samchui.com

all_licenses: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/4.0/


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What is the physical experience of passengers in the cabin during an Emergency Descent?
I am not a pilot so please forgive any misuse of aviation terminology; I'm merely a person who is working hard to overcome a sudden and intense fear of flying.
While working through a course to overcome this issue, the topic of emergency descents came up. I learned the purpose of these and the actions the captain / copilot must take, but the experience of the passengers was not discussed.
If a commercial jet is at 40,000 feet and experiences rapid depressurization, and the captain decides to make an emergency descent to 10,000 feet, what does this feel like for the passengers? Is it like being on a rollercoaster with a 30,000-foot drop instead of the typical 100-foot? In other words, do passengers experience freefall / weightlessness?
Do unbuckled passengers go flying all over the cabin? Is all unfastened material thrown across the cabin? What happens to crew who cannot make it to a seat fast enough? How common are these rapid descents in commercial flying?
I imagine this is probably the most terrifying experience a person can have, if it's anything like plummeting like a meteor.

Emergency descents were one of the more fun maneuvers during training, might have a different view of them as a passenger though. Unless the aircraft is on fire, "emergency descents aren't that steep. The pilot has about 15 minutes to get down to thicker air before the passengers oxygen runs out.

No nothing like that.  If there was what is usually called "explosive" decompression, like a window blowing out, there will be a loud BANG, maybe a short howling sound as air rushes out, possibly fog will form in the cabin, and you will feel the air pressure in your head trying to get out until it equalizes.  It might make your ear drums hurt until it does.
The air will be chilled by the temperature drop caused by the pressure drop.
The big surprise is you can't feel the "thin" air when you breathe (you'd have to be exerting yourself - just sitting still, you can't tell).  The masks drop and at that altitude you have maybe 15 seconds to get one on before you pass out.
At any rate, explosive decompressions are pretty rare and emer descents are usually triggered by more gradual situations, like a slow loss of cabin pressure, or a windshield that suddenly cracks (although with several laminations and it normally won't fail totally - still, it'll scare the flight deck crew pretty good).
As far as the plane goes, you'll notice the thrust going to idle, the airplane decelerating temporarily, and you may feel the sensation of the nose pitching down to maintain speed, maybe a slight cresting-a-hill-in-a-car reduction in G sensation momentarily, but that's about it.  Nobody floating around the cabin.
When the capt gives the command for emergency descent, whatever the reason, (after they get their masks on and establish communication with each other) the procedure is to pull the thrust to idle right away, spin the altitude select knob for the autopilot to a lower altitude, and set the autopilot to "speed" mode, which tells the autopilot to pitch the nose to maintain airspeed (if the pilot were to shove the nose over manually, it could result in going zero or negative G and making people float around, but the procedure is usually done using the autopilot, which does the pitch over gradually, so that shouldn't happen).
The autopilot will start to pitch the nose over to keep the speed up since the thrust was taken away, so there will be the initial deceleration from pulling power, then a very gradual acceleration in the descent. If the plane has "flight spoilers", the kind that can be extended in flight, they will also be set fully out, so there'll be the rumble from that, and possibly the capt may drop the landing gear to increase the descent rate, depending on the speed and the airplane.
As it starts to pitch over the pilot will dial the speed to maxmimum operating speed if he/she is confident there is no major structural damage.  If unsure, you are supposed to stay at the speed you were at when whatever happened happened.  The most unpleasant part will probably be the noise level in the cabin if there was a rupture of some kind.  Even if there is nothing blown out, when descending at maximum speed the overall noise level is pretty high, a lot louder than cruise.
They'll be heading for 10000 ft and it'll only take a couple of minutes, so it'll be over pretty fast.
Thing to keep in mind about those plastic cabin windows (the structural outer panel, not the one next to you with the tiny vent holes at the bottom). If it's 8" x 10", there is over 600 lbs pushing on it with the cabin pumped up to 8 psi.  Sounds dicey.   But the acrylic plastic is very thick and the window actually has roughly the same strength as the (much thinner) adjacent sheet metal, so it's actually subjected to a fraction of the stress it's able to take.


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How to adjust the center of gravity of an aircraft if not level when being weighed?
how do you adjust the center of gravity when weighing an aircraft in not-leveled position using platform scales under the nose and the main gears, and dropping a plumb bob to mark the gears locations.

I drew in the approximate location of the screws used to hold a level while leveling the Cessna Cardinal for weighing.

When my plane was weighed, they had to lower the nose by deflating the nose strut some, and raise the main gear, to get the aircraft to the level position indicated by the manual, with a level across the leveling screws on the side of the fuselage. I don't know that there is a procedure to weigh it in a not-level position.

Is the question how do you level it, or how do you adjust the center of gravity?  Those are two different things.

Doesn't lowering the nose and raising the mains adjust the center of gravity? I would expect that to move it forward. Along with having full tanks, which are in front of the main gear.

No it doesn't matter how you drop the nose or prop up the main gear as long as the airplane's longitudinal axis is level.

sorry, I think I didn't ask the question correctly. I am tasked with weighing a fixed wing ATR 42 cargo aircraft. The leveling points on the cargo door of the aircraft are damaged. So I need to weight the aircraft without leveling. I  am going to find a place on the aircraft to use an inclinometer to read a pitch angle and then generate some math to adjust the cg. I just was wondering if anybody has done that kind of weighing.

As Crossroads shows, the data provided for W&B calcs is based on a level fuselage. If the fuselage is not level, you would need to do some math to calculate adjustment factors for the wheel weight values, and to do that you would also need to know where the vertical C of G is.  Good luck with that.  So there's no work around that I've ever heard of, and you're kind of stuck with leveling the aircraft properly.

Just to put it into perspective:
$Sine$ 4 degrees × 100 = 7 % grade

$Cosine$ 4 degrees x 100 = 99.75 % of perfectly levelled scale.

So unless your scale is on a fairly steep mountain grade, your main concern, if any, is to level the plane.  It would seem to be much easier to use a block and a bubble, with an appreciation of the fact that the "ball" inclinometer instrument in your aircraft works exactly in reverse, seeking the "lowest" point instead of the highest.
The alternative would be plum-bobs and a lot of math.  However, with spreadsheets such as Excel, it would make an interesting project.
Keep in mind pitch (within reason) does not affect center of gravity.  The torque arms will shorten proportionally.  The plum-bob tells you where to set the scale.  Formulas in your spreadsheet can be validated by checking results at various pitch angles for each aircraft.
Validation is highly recommended for an application as crucial as CG.


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What is the relation between velocity and rate of pitch?
I saw the answer to airspeed and rate of turn but this I think isn't what I want. I've seen it many times that aircraft will pitch up much slower when at higher speeds particularly fighter aircraft. What is the mathematical relation of these two characteristics if any?
I am trying to make a game about fighter aircraft and want to find a realistic solution to this problem.

A constant speed pull-up is basically a circular motion, albeit in vertical plane:
$$q=\frac{(n_z-1)g}{V}$$
where $q$ is pitch rate, $V$ is true airspeed assuming no wind, $n_z$ is the load factor (lift to weight) and $g$ is gravitational acceleration.
The amount of load factor is limited by the airplane and its flight condition. As you can see, the faster the speed, the smaller is the obtainable pitch rate for a constant load factor.
Note: a constant speed pull-up is a theoretical idea that only approximates a pull-up at the very beginning, before the airspeed begins to decay. But the conclusion is valid.

is g 9.81? or is it like g's where it can increase or decrease? Or better said, downward acceleration in circular motion. And the load factor I'm assuming is the weight in kilos of the aircraft?

@Kiyo There is no weight or mass in this. g can be in whatever unit you want, as long as it jives with the rest of the equation (i.e. must be consistent with the units of q and V).

I see but when what exactly is the load factor? Where could I get this number from?

@Kiyo See https://aviation.stackexchange.com/questions/46287/what-is-the-definition-of-load-factor-and-how-do-you-apply-it. It's the instantaneous lift over weight.

I see, thank you. One last thing. what units is q in? Ideally, I'd like to be able to put this into torque.

@Kiyo Per my last comment, they "can be in whatever unit you want, as long as it jives with the rest of the equation". Torque and q are apples and oranges.

q in rad/s, g in m/s$^2$, V in m/s. n has no unit, it is a constant.

If the aeroplane flies straight and level, and the pilot pulls on the stick in a step input, the elevator deflects => increased Angle of Attack => increased lift. The aeroplane climbs while continuing to rotate backwards, trading kinetic energy for potential energy.

In the situation depicted above, the centripetal acceleration is $V \cdot q$, and hence: $$N - W =  m \cdot V \cdot q$$. So for a given elevator deflection and therefore increase in N, at higher V the achieved q is lower.
As a guideline for maximum n = N/W, from this wiki page:

For transport category airplanes, from -1 to +2.5 (or up to +3.8 depending on design takeoff weight)
For normal category and commuter category airplanes, from -1.52 to +3.8
For utility category airplanes, from -1.76 to +4.4
For acrobatic category airplanes, from -3.0 to +6.0
For helicopters, from -1 to +3.5[7]


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Are helicopter pilots required to use GPS navigation?
Some helicopter crashes are caused by a disoriented pilot. This would presumably be impossible with a GPS with terrain maps. Are pilots required to use a GPS when flying a helicopter in the US?

Disorientation is often not about losing track of where you are but about losing track of where you're going, and which way you're pointing. This is very much possible even with a GPS.

a lot of helicopters (and aircraft) don't even have GPS equipment and are perfectly legal to operate.

To add to pericynthion's point, here's an accident report about a GPS-equipped helicopter CFIT, IMC at night: https://www.ntsb.gov/Investigations/AccidentReports/Pages/AAR0602.aspx

I haven't read the report in detail but the chapter "Aids to navigation" simply states "not applicable".

There is a cockpit diagram on page 43 of a representative aircraft which clearly shows "glass cockpit" avionics commensurate with the statement "GPS with terrain maps". There's also a minor reference to the aircrew programming GPS on page 45. I imaging the reason that the Nav Aids section is "N/A" is because it wasn't a factor in the accident, but, your hypothesis that the moving map might prevent the crash is in this case not true. The report finds the crew failed to recognise instrument indications that they were descending into the sea. I think is also relevant to your hypothesis.

GPS doesn't help you know which way is up.

Not only is it possible to become disoriented with a GPS installed, but the GPS could actually contribute to that disorientation. Aviation GPS systems are a bit notorious for being difficult to program. Pilots can and do easily get disoriented when they start fidgeting with one, get fixated on it, and end up ignoring all the other flight indications.
Orientation and situational awareness aren't technology problems; they're training problems.

As said above, GPS won't keep a pilot from being disoriented.  Keep in mind that helicopters are routinely used in extremely dangerous operations where the pilot may be unfamiliar with the location and flying dangerously close to multiple objects at the same time, not to mention the terrain.   Wires that hold up antennas are invisible.  It is a tremendous workload for the brain.  In these complex situations getting disoriented is always a scary possibility.

A disoriented pilot may include pilots who are disoriented with respect to attitude.   GPS does not provide attitude information, although it could be inferred to a degree.
A Ground Proximity Warning System (GPWS) uses GPS data and digital elevation models (terrain) to predict terrain collisions.  Such a device gives warning when an aircraft trajectory is likely to collide or nearly collide with terrain.  These systems are common on fixed wing and rotorcraft.
In 2002 the US FAA mandated TAWS or GPWS for all turbine airplanes having 6 or more passengers.  I am unaware of a similar requirement for helicopters, but I seldom fly helicopters, particularly larger than a 206.
For helicopter flights there is only a requirement in the US to have maps or charts suitable for the navigation used. If VFR these could be Helicopter charts, terminal charts, sectionals, WAC/ONC, even instrument charts.  Old man Jeppesen just made notes on a notebook, and later marketed them.
To specifically answer your question, there are no generalized requirements for GPS or moving map GPS in helicopters.
And as a relevant footnote, GPWS systems have been available at a relatively low cost for general aviation aircraft for well over 20 years, which would provide terrain warning, with verbal audio in the pilot cabin.

No, pilots are not required to use GPS maps or moving GPS while flying helicopters or fixed-wing aircraft.
There are some approaches to landing that require GPS (called RNAV) approaches, but the airports that they serve usually have other types of non-precision approaches as well.
Helicopters have specialized maps that are called Helicopter Route Charts that are supposed to provide additional information relevant to helicopter pilots. These charts may be carried in paper form, although newer aircraft also have them as electronic moving maps. The pilots may also (but are not required to) be using electronic flight bags with moving maps/terrain/synthetic vision.

RNAV (be it B or P) is also required on certain airways, departure and arrival procedures (even when followed by a traditional instrument approach) and in certain airspaces (when IFR, of course). The days of VOR2VOR are long gone and inertial navigation can only guarantee certain precision.

Yes, to fly some routes/approaches you need specific equipment, like a GPS, but the pilot could also pick an approach/airway that doesn't have those requirements.

Well, according to European Civil Aviation Conference enroute operations above FL95 require BRNAV (RNP5). Countries may designate non-RNAV routes in the lower airspace but there is no such route in our country. I assume one could get a clearence for DCT below FL95. Many TMAs require PRNAV (RNP1).

@VladimirF The question is tagged "FAA", so I wasn't considering any other countries in this answer.

GPS is never required in any aircraft. No navigational aid/equipment is required in aircraft flying VFR (Visual Flight Rules) or Daytime Special-VFR except for a magnetic compass. Nighttime Special-VFR and all IFR (Instrument Flight Rules) require navigational equipment specific for the flight undertaken and/or the Instrument Procedure flown. This may or may not mean GPS, VOR, DME, LOC, and/or ADF may be required. It depends on the circumstances. Even the magnetic compass is not considered a navaid in an aircraft. It is technically a flight instrument.

authors: Vladimir F Героям слава


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LEFT RIGHT PROBLEM
I am writing an article about the LEFT RIGHT CONFUSION which has resulted in many accidents where pilots have shut down the wrong engine in this confusion. Many people have a real problem in deciding between RIGHT and LEFT though not much attention has been given by aviation medicine on the subject. I wonder how many of you or people you know have this problem and it wont be wrong to assume that under stressful circumstances like on board emergency a pilot may go for the wrong engine like it happened in Trans Asia Flight 235 on 04 Feb 2015 where the pilot switched off the only working engine after encountering EFATO.

That confusion caused a crash of a passenger plane, a twin-engine CN-235, near Malaga, in 2001. https://www.1001crash.com/index-page-description-accident-BM_CASA235-lg-2-crash-121.html

Hi @BirenderYadav, what exactly is your question here?

Welcome to Stack Exchange! This site isn't really set up for survey-like questions like "I wonder how many of you or people you know have this problem"; we're only good at answering questions that invite answers based on research and expert knowledge.

Batt Masterson once said about gunfighting that is was "the art of taking your time in a hurry".  This is pretty applicable to handling emergencies in flying and, in particular, engine failures in a multi-engine aircraft.  Engine failures during low speed operations are very serious and you must take timely, prompt - and CORRECT - action to prevent it from turning into a tragedy.
The first course of action is realizing that the art of flying twins is good, periodic, recurrent training on handling engine failures.  This MUST be a part of any competent Biennial Flight Review in a twin and an instructor MUST hold the student to being able to handle engine out problems to at or above that required for a practical test for his/her grade of pilot certificate.
The second line of action is develop an emergency plan as part of your preflight and stick to that if something goes wrong.  This will vary depending on the airport you are departing or arriving at, aircraft loadout, atmospheric conditions, etc. and has to be a part of a solid preflight process.
In regards to avoiding killing the good engine inadvertently, remember the mnemonic DEAD FOOT, DEAD ENGINE.  When an engine failure occurs, the aircraft will try to yaw sharply in the direction of the dead engine and opposite rudder pressure will be needed to keep the nose of the aircraft tracking straight.  This means one of your legs will be doing a lot of pushing on one of the rudder pedals while the other leg is relaxed or 'dead'.  The failed engine will always be on the side of the 'dead' leg or 'dead' foot.
This process should be incorporated into a cockpit flow for engine failures in critical situations.  Once you note which engine you suspect of failure from rudder pressure, next VERIFY THIS.  YOUR CHIEF INDICATIONS OF A FAILED ENGINE PAST THE RUDDER PRESSURE IMBALANCE WILL BE DROPS IN 1) FUEL FLOW AND 2)  EGT/TIT TEMPERATURES.  NEVER RELY ON MANIFOLD PRESSURE INDICATIONS, PARTICULARLY IN NORMALLY ASPIRATED ENGINES!
Finally test this by pulling each throttle back individually and noting whether there is a change in engine performance in doing so.  The throttle on the failed side will not cause any change but the throttle on the good engine will produce power changes when moved.
With this information, we repeat carefully with words and pointing "DEAD FOOT, DEAD ENGINE, DEAD ENGINE GAUGES, DEAD PROP FEATHER.  Verify you hand is placed on the dead propeller control, then pull this to the feather position.  Finally verify your hand is on the dead engine's mixture and pull this only when the other actions are complete and the airplane still appears to have power on the good side.

"DEAD FOOD"?   I'm editing that for you.

Overall good answer, but the part about throttling back needs to revised. You don't "experimentally" throttle back each engine. When you start the shutdown drill phase, the first step is to throttle back the dead engine you've already identified. When you do this, you evaluate the result, as you said but hopefully the result is no change, and you can continue on to feather the prop yadda yadda yadda. If you're in a piston twin at gross weight on a hot day, climbing at 150 fpm at blueline speed, that last thing you want is to reduce power even for a second, on the only engine keeping you alive.

@JohnK I think we’re more or less describing the same thing.  However I’ll revise to include pulling the throttle to ideal once I identify it for the failed engine only AFTER I individually change power settings on each engine.

The important thing is it's balls to the wall until you are ready to start securing and feathering.  Dead engine throttle back, confirm that's the right one (by the airplane not responding), then feather, fuel, etc.  You don't touch anything before that step, except to make sure everything is fully forward.

and @abelenky, given some photos I recently had shared with me by a CFI friend of mine in CA, whose client hit a deer when landing his SR-22, you really CAN have Dead Food, Dead Engine!

@JohnK This post is more focused on verifying which engine has failed as opposed to a complete engine failure procedure in a twin.


text:
Who opens an airliner's door first after it lands?
I know this is a trivial question, but since I always sit near the back of the plane, I can't see what happens up in the front. Whose responsibility is it to open the cabin door once the aircraft has pulled up to the jet bridge?

It depends on the type of aircraft. But USA today/Ask the Captain gives a nice explanation:

Question: I was wondering what takes so long to get the exit door open after we land. Could you please go through the steps?

Answer: Once the airplane arrives at the gate and is properly chocked, the captain shuts down the engines and switches off the “Fasten Seat Belt” light. The anti-collision light (this is the red blinking light on the top and bottom of the airplane) is switched off, informing ground crew that it is safe to approach the airplane.
Ground crew members then begin to position the jet bridge, aligning it with the door.  Once the jet bridge is mated to the aircraft and the cover is in place, the ground agent either opens the door or taps on it for the flight attendant to open it. This varies depending on the type of airplane to minimize the chance for an inadvertent slide deployment.
Once the door is initially opened, the ground agent assists in fully opening it and ensuring that it is locked in the open position.
This is the generic procedure for most jet airliners. Regional or smaller airplanes have a slightly different procedure as their stairs are built into the door.


text:
How significant is the static buildup on the blades of the main rotor? And some questions related to this
1) Usually, a static charge is built on the helicopter when it flies through the air. If a person touches the helicopter before it touches the ground can that person get electrocuted because of the potential difference? 2) How significant is the static buildup on the blades of the main rotor? Are they grounded to the airframe of the helicopter? 3) What happens if a person jumps right into the helicopter while it's hovering, without completing the circuit with the ground? Like in the Matrix (1999), when Morpheus jumps off a building into a helicopter?

I don't have any evidence, but anecdotally I have been warned that grabbing an under-slung load before it touches the ground can hurt. I have never felt a shock, but have only worked under smaller helicopters. I have not been told this shock is dangerous, but again this is anecdotal advice from old guys at work, I would be interested in how much charge can accumulate on a large one.

1) Technically, yes, it's possible to be electrocuted by a static discharge. The chance of it happening is so vanishingly small, though, that it's not generally a concern. A helicopter simply doesn't have the capacity to store enough electrical energy to be a direct danger. The real threat from static electricity is that it can potentially ignite a fuel spill, or something of that nature.
2) Depends on how you define "grounded". The linkages are coated with grease, which typically has a pretty high resistance. This means that low voltage (where "low" in this case can be up to 500 volts) across the junction probably isn't going to generate a meaningful current. However, static charges routinely exceed 10,000 volts, which isn't going to be affected by a thin layer of grease. So, you can generally consider the whole plane to be at the same potential.
3) Same thing that happens if you're on the ground and touch it. When you touch metal, you'll get a spark, yank your hand back, probably shake it a few times if the spark was particularly bad, and resolve not to do that again.

That's a great explanation, but regarding question 3. When on the ground the static electricity is low, if you're talking about touching the surface of a vehicle, which is most likely to have more or less conductive landing gear or tires. That's why I don't exactly understand why a person attached to a hovering helicopter can grab a hand of another person who jumped from a building, being at different potentials they can be both safe and sound in real life, or may be through the air gap between the two people, that is narrowing as they get closer, the potential difference neutralizes?

@PaulJ. Think of two water tanks with a different level of water in each. Connect them, and water will flow from the higher tank to the lower until they're equalized. That's what happens when two objects at different potentials are connected, a current flows through the connection until they equalize. That brief current flow is what generates the spark, what stimulates the pain receptors, and what can cause damage if it's big enough. It doesn't matter why the electricity flows, it just matters how much, and a helicopter doesn't have the capacity to generate a dangerous amount of current.

note on #3, modern helicopters are electrically bonded from the blade tips to the skids for lightning protection.

It should be noted that, when a helicopter is involved, even a moderate to mild shock can be fatal or debilitating if, as a result of the shock, you fall out of the helicopter. Sure, at best, you get some bruises and scrapes, but the worst can range from spine fractures to falling on the ground and then getting bashed to death by the helicopter's wheels or skids because helicopters tend to move around quite a bit, and suddenly, when landing.


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What is the westerly index?
The westerly index, appears in a CPL syllabus but I can't find much info on it, I assume it is a measure of how systems move from west to east in the southern hemisphere.
How exactly is it calculated, and what is it's purpose and significance for aviation?

Related question on Reddit

I'm not sure, but I'd hazard a guess and say it's related to the Westerlies, prevailing winds from west to east between 30 and 60 degrees lat.

Weather systems move from West to East in the Northern hemisphere too.

"The Westerly Index (WI) is defined as the proportion of days per month with prevailing wind coming from the west in a four point quadrant (wind blowing between 225 and 315 degrees)". Used to study the variability of Westerlies. It is related to a defined area, e.g. south hemisphere, Asia, etc. Here for the English Channel. WI data set example for the same area.


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How do STOVL and V/STOL aircraft differ?
Wikipedia's page for STOVL begins with

A short take-off and vertical landing aircraft (STOVL aircraft) is a fixed-wing aircraft that is able to take off from a short runway (or take off vertically if it does not have a heavy payload) and land vertically (i.e. with no runway).

while V/STOL is

A vertical and/or short take-off and landing (V/STOL) aircraft is an airplane able to take-off or land vertically or on short runways. Vertical takeoff and landing (VTOL) aircraft are a subset of V/STOL craft that do not require runways at all. Generally, a V/STOL aircraft needs to be able to hover.

On both pages, it is stated that the respective type should not be confused with the opposite one.
Essentially, it says STOVL is not the same as V/STOL and vice versa.
However I have trouble understanding where the differences lie between them.
I was about to clear it up on my mind when I thought STOVL definitely requires some (short) distance for the aircraft to clear before taking-off, while V/STOL aircraft are simply able to start hovering in order to take off. But apparently, it's possible for STOVL aircraft to do so if they don't have heavy payload.
What am I missing here? Both are supposed to be able to land vertically so I suppose there's either a difference on the take-off or if not, there are differences on the wing structure/geometry of each type of aircraft.

Just from the text itself (guessing, so not an answer), it looks like VTOL must be able to take off vertically at max weight, whereas STOVL can only do so when light, and STOVL must be able to land vertically, whereas STOL does not.

The distinction between STOVL and V/STOL is largely a matter of naming. As it stands, no one has operationally deployed a fixed-wing aircraft that can land vertically, but cannot take off vertically. All modern aircraft referred to as STOVL or V/STOL are, in technical terms, VTOL (as well as STOL) capable aircraft.
Designating them V/STOL was a matter of emphasizing that they can combine the higher useful load advantage of STOL operation with the flexibility of VTOL operation. The term "STOVL" is a further attempt to distance away from the notion of VTOL as low-performance. In the F-35B's case the justification is that a conventional takeoff ramp is still required for practical operations, but a simpler and more compact vertical recovery can be used. Actual operational practice may dictate outright STOL operation to be preferable.
In short: As used presently, these terms do not carry an inherent technical meanings. It's just practice, and in some cases can even be thought of as public relations choices.

To be technical, an autogyro can land vertically, but can't take off vertically.  Of course the terms STOVL and V/STOL are generally applied to modern military aircraft, which an autogyro isn't.

@rclocher3 Actually some Autogyros are capable of vertical takeoff.  It’s referred to as a jump takeoff and is possible depending on the inertial characteristics of the main rotor as well as having a prerotator capable of attaining a significantly higher rotor RPM than that used for flight.

I searched for online videos of autogyros doing jump takeoffs, and found this one.  Interesting!  Thanks @CarloFelicione!

@rclocher3 yup, that’s a jump takeoff!

To put it simply, it's about the ability to take of vertically at any mass:
V/STOL aircraft can take off fully loaded (max. operational weight).
STOVL aircraft does not have sufficient engine thrust to take off vertically at any weight, and at higher T/O weights it must have some lift from wings to assist the T/O. It may be able take off vertically at less than max. T/O weight.

While this could be a valid definition, in practice all VTOL or V/STOL or STOVL fixed-wing aircraft ever in service can/could take off vertically at some weight, and none could take off vertically at max operational weight.

You’re right.  V/STOL and STOVL are one in the same.
It would probably be better to consider the term STOVL more of a mission profile than an aircraft capability.
All powered lift aircraft eg Harrier, F-35B, etc. are capable of vertical takeoff and landing within certain weight limits.  They are not capable of vertical flight outside this.
The directed lift system on these aircraft do allow lift to be augmented by vectored thrust, allowing for short takeoffs and landings this way at greater weights.
The upshot of which is that, for certain operations, it makes more sense to a jet like that to execute a short takeoff when heavily loaded ie fuel + weapons load, fly a mission, thence, once fuel and stores are significantly depleted below max hover weight, then make a vertical landing.  The profile works well for ops from small helicopter carriers or unpaved, short airstrips where CTOL operations just aren’t possible.
Now a STOVL or V/STOL capable aircraft should not be confused from a pure VTOL aircraft eg MACAIR DCX, SpaceX Falcon 9 Booster, DJI Mavic, etc., which cannot accomplish short field takeoffs and landings.  Even a helicopter would be considered a V/STOL aircraft as it is common for some ops to do short field takeoffs and landings when operating at density altitudes above the maximum hovering altitude for a given takeoff weight.

V/STOL and STOVL are as much operational modes as types of aircraft.
In V/STOL, the aircraft is envisaged as sometimes taking off and/or landing vertically and sometimes using a short runway, adapting its mode of operation to the job in hand. Some hybrid and compound rotorcraft can operate in STOL mode as well as VTOL.
In STOVL, the aircraft is envisaged as taking off from a short runway, either because it is heavily loaded or it is based there, and at the end of its flight landing vertically, again either because its fuel load is now so light that it can hover, or the destination demands vertical descent. The UK Sea Harrier fleet typically operated in STOVL mode, taking off up a ski-jump and landing vertically.
Some aircraft are optimised for the one mode or the other and described accordingly, others are designed for both modes and the choice of description is a marketing or editorial one.
Weirder non-standard variants are sometimes cooked up for specialist modes or sub-modes such as VTOSL or VTOCL (conventional landing) for a vertical-launch point-defence interceptor, or STOV/SL which speaks for itself. Thankfully, none has stuck.


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Does the large 6-pointed star on a sectional warn of dangerous radar facility?
I cannot find an explanation for this symbol on the chart legend.
Example: near BAB, Beale AFB, on the San Francisco Sectional Chart, does it warn of the dangerous radar facility?

Source: skyvector.com

Note that the callout box is re radar hazard is connected by a dotted line to the large star.  Could be a good thing to note in the question or in an answer.

There are two stars:

The large multi-pointed Star (6 pointed in this case) is described in the Aeronautical Chart User’s Guide as an indicator of High Energy Radiation Areas. The Precision Acquisition Vehicle Entry Phased Array Warning System (radar) may be intense enough to damage avionics. Or, at least cause them to give false readings.
The blue star with the clear circle in its center indicates that the airfield has a rotating light beacon and the beacon’s location relative to the field. The beacon’s location can be used to differentiate one airfield from another while in flight at night.

Reference: Aeronautical Chart User’s Guide

No, not that symbol, it is a very large 5 point star with a 6 or 9 within it. It is adjacent to a caution box which warns of USAF PAWS radar hazard to aircraft.  I think the star calls attention to the PAWS, but an not certain,

Wow. I had to look closely at the small 5-pointed star to realize that it had a "clear circle" in the center. To me, it looked like a small outline of a 5-pointed star, much like the large 6-pointed star is an outline.  I see it now.

Then there's the third star, to the right of the control tower frequency, indicating that the control tower is part-time.


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Why are engines angled toward the fuselage?

  Source: twitter.com

I saw a picture of a 777 from overhead and I was wondering why the engines point "in" to the fuselage, rather than being parallel to the fuselage.

related

they look like they are pointing out to me... but then, I guess I'm looking at it from the point of view of which way the exhaust is going.

It's called 'toe-in' and it's done basically to match the local airflow which is slightly divergent (heading outboard along the underside of the wing). If the engines were mounted exactly parallel to the fuselage, they would be moving slightly obliquely through the local airflow and therefore incurring extra unnecessary drag.
Another view of a 777 showing the engine toe-in:

  Source: airliners.net

The Boeing 747 has a 2 degree toe-in on all four engines, as can be seen in the following plan:
[Key: BBL 0 = Body Buttock Line zero, the centreline of the fuselage; WBL = Wing Buttock Line, parallel to that centreline; NAC BL0 = Nacelle Buttock Line 0, the centreline of the nacelle body]

and in this photo:

For rear-engined aircraft, the opposite is true: the engines are mounted slightly' toe-out' to match the local airflow which is converging around the tail of the aircraft.

This is interesting, as I would expect the local airflow to be less inward when you go from the inboard engine to the outboard engine (as there is less influence of the body). This would lead to a smaller angle in the outboard engine. Anybody has an explanation for that?

If it costs more to make them different than it does to make them inefficient, that's what will be picked, as with everything else in large-scale manufacturing and in aviation, likely the reason. @ROIMaison

@ROIMaison The swept wing is also a major contributor in outboard flowing airflow. If the aircraft uses a straight wing instead then we may see near zero spanwise flow on the outer part of the wings

Wouldn't the wings flexing backwards slightly due to drag also be a factor? That would make the engines in flight be slightly more outwardly-rotated compared to when they're on the ground.

@nick012000 The forces on the wings would result in a forward deformation between the fuselage and the engines; the engines push forward and the fuselage is dragging behind. However, due to the length of the chord and the way the wings are laid up, they are VERY rigid in the fore/aft plane. Wings flex up and down, not fore and aft. As to the outboard engines, Slebetman is correct; swept wings induce a significant outboard flow at the leading edge. Where the engines are mounted the outboard flow is reduced but still present.

@Nij That doesn't sound like a good reason. 1. They're different mounting points on the wing regardless. 2. If it makes that big of a difference to toe in the interior engines by 2 degrees then surely the financial efficiency of adjusting the outer engines would be worth it if the physics demanded it. 3. This isn't like some trivial interior ammenity where they can pickup economies of scale cost savings. This is the engines.

But why is the entire engine canted, instead of just the inlet? For example the MD-80 engine has the inlet tilted a few degrees, but the exhaust is parallel to the body.

@Bret Huh, I hadn't considered the forces the engines would put on the wings. Wouldn't the lift/induced drag forces be greater than the thrust force, though? I know that the engines don't have enough thrust to carry a big aircraft alone, so the overall lift/induced drag force would be a lot greater than the engine thrust.


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Can tailwind produce reverse-thrust?
I know that the answer will probably be no, and this question might sound silly, but I wanted to think outside of the box.
If engine air intake comes from the front, and is pushed out through the back, what happens if the tailwind is so strong that the opposite happens, and the air is pushed in through the back of the engine? Will it create reverse-thrust? Is this dangerous?

Remember that an aircraft moves in relation to the surrounding air, not in relation to the ground. If there is a tailwind, the aircraft will have the same speed through the air, meaning a faster speed over ground. Air will never mode backward over a wing.

Related: https://aviation.stackexchange.com/questions/65219/

I suggest that you make that an answer.

If you're ever in a situation where the tailwind is so fast that it's actively pushing the exhaust back into the engine, I'd say we've got far worse things to consider (on a grand scale)...

Related: Why do gases in the combustion chamber only flow one direction to the gas turbine in a jet engine?

Also: How do jet engines work?

The ambient air will only move through the engine from back to front when the aircraft is on the ground. Even then, the engine would have to be off for that to happen. Otherwise, the thrust from the engine would be too great. Even the electric or air starter should have enough torque to overcome the ambient air pressure before fuel ignition is achieved. Go out to an airport ramp during a windy day. Some turbine engines without inlet and exhaust covers will windmill.
In the air, a tailwind only adds to the forward velocity of the airplane as a whole. The airplane is traveling through the airmass irrespective of the ground. Think of it like a child’s balloon floating in a minivan on the highway. The balloon does not know nor care that it’s doing 70-80 mph.

Only when on the ground, and at the inlet end, where a strong wind from  the side and behind could influence the engine's ability to draw air in the front at low power settings.  Some engines have limitations or restrictions on how power is applied during takeoff in strong tail or quartering winds, because of initial flow disruptions at the front from air having to make a sharp corner into the intake, where large thrust increases could cause compressor stall/surge.
You might have a procedural requirement to advance power to some intermediate setting, then let it stabilize there before going the rest of the way to takeoff thrust. The GE CF-34 has limitations like this, where winds above a certain speed and off angle a certain amount require a staged application of thrust.
Once you're in the air, there is no such thing as a tailwind as far as the airplane is concerned except insofar as tailwinds add to ground speed.  For the engine, it's all coming from the front as soon as you are moving with any significant airspeed.

Whereas an airplane with a tailwind is moving at it's true airspeed (or EAS) plus the wind value, the airplane never actually experiences the tailwind, other than as expressed by its speed over the ground.
An airplane on the ground during engine start, with a tailwind, can experience a slightly elevated start temperature up through about 35% during the engine start.
Reverse thrust isn't blowing engine exhaust or air out the front of the engine.  it's the act of deflecting fan airflow out the sides of the engine.  Reverse thrust is a combination of drag rise around the engine nacelle, and a rise in intake drag and internal drag in the engine while the thrust is re-directed.  A tailwind can't duplicate that, and has no effect on thrust in cruise flight.


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Why isn't there more green space on the apron?
I feel like it's a waste of concrete or asphalt. Can't they just leave some areas that don't get taxied on as grass? If taxiways are designed with green spaces in between them, why can't we see much grass on the apron?

Image source: https://www.internationalairportreview.com/article/108788/toronto-pearsons-story-of-growth/

The same reason that highways have grassy medians and shoulders whereas parking lots are generally a solid expanse of pavement. One is optimized for moving vehicles in an orderly and efficient fashion from one place to another and the other is optimized for packing stationary vehicles as densely as possible around that destination.

Though it is quite common to see parking areas with "medians" between lanes of parking that have trees or other landscaping growing in them.

@jamesqf Small islands are mostly decorative and reduce efficiency. At worst, they attract birds that poop on your car while providing no real shade in return. Larger islands separate parking areas from through lanes, which just reinforces my point.

I made no claim to their efficiency or lack thereof.  I state that they exist, simply as a matter of observed fact.

@jamesqf, at an airport, all those medians are taken up by the terminal concourses to which the air bridges attach.

Grass, gravel, and dirt dirt between movement areas like runways and taxiways makes sense because the movement of vehicles and persons in those areas is so highly controlled and restricted. Aircraft have to remain within certain boundaries. The same can not be said in non-movement areas such as ramps and aprons. Coordinated and choreographed are better terms to describe movement in non-movement areas. Aircraft and vehicles can and really must be able to move in any direction necessary on the ramp. Most aircraft can not move in reverse without external assistance. And, the ramp is much too dynamic of an environment to overly restrict movement.
Grass, dirt, gravel and other soft surfaces would only work for movement areas for very small aircraft. It can not easily bear the weight of large aircraft. It would be very weather dependent for even small aircraft. Any lingering water from rain may make it impossible to move an aircraft with normal tricycle landing gear without bogging down the wheels. Whether you can park an aircraft on such porous surfaces would be dependent on local ecology rules and laws. Some areas make it illegal to park any motor vehicle on grass.
I am not an airport engineer. Based on general experience with concrete and asphalt, I can see why you would use concrete on areas where vehicles are sitting still and asphalt where vehicles are moving.
Concrete is a very chemically resilient substance. Oil and fuel spilled on concrete will not erode and/or dissolve it as quickly as it would a petroleum based substance like asphalt. That is why gas/petrol stations all have concrete parking lots. In some countries, any area where vehicle fueling takes place are mandated to have concrete pads. Vehicles sitting for periods of time will leak fuel and oil. Parked aircraft fuel tanks are usually full of fuel to prevent condensation and water accumulation. They are vented to account for expansion. They will leak.
Concrete is very strong, yet very brittle in comparison with other materials. Breaks and cracks can form due to seasonal heating and cooling. That is why concrete slabs are built with expansion gaps/joints. Extreme temperatures can cause even more damage. Water trapped in the porous concrete or in between crack can expand when frozen and break the concrete. Shifts in the underlying earth due to the moisture level of the soil varying throughout the year can severely damage concrete slabs.
Asphalt is a very impact and expansion resilient substance. It can handle the rolling pressure of moving vehicle wheels. Just as important, it can handle the pressures of seasonal expansion and contraction better than concrete. When asphalt is damaged from use or buckles from expansion, it is quicker and easier to repair  than concrete.
Asphalt’s chemical resistance is not so great. Asphalt is a petroleum product. Petrochemicals like fuel act as a solvent. Like chemicals dissolve like substances. For instance, you can dissolve an asphalt speed bump on a street with a few gallons of diesel over time. Airplanes spend a smaller amount of time on asphalt runways than they do on concrete aprons.

I honestly see no spots where grass wouldn't be in the way or limit the usage of the gate in the photo in the question. Different aircraft types require different amounts of space around the gate, patches of grass could limit what aicraft types a gate could service.
Even if you design the gate to be able to handle all current aircraft, a new model could be designed in a year or five that would revolutionize the air industry with a new design which wouldn't be compatible with your patches of grass. Sure, you could redesign the apron, but why bother when you can avoid the issue in the first place?
Most airports store various bits of equipment around the gates; having patches of grass would make the apron less flexible, and more costly to maintain. If nothing else you'd need to employ someone to cut the grass and give them some gear to do it with; depending on where the grass is located you might not be able to use the same (huge) cutters used other places in the airport. Depending on the size of the patches of grass it might also impact gate availability which is something you don't want.


text:
What happens if an aircraft goes above maximum altitude?
We all know what happens when a plane goes overspeed... but what happens when a plane goes above their max. altitude? And why are planes certified maximum altitudes anyway? Is it because there won't be enough lift to keep them at that altitude? I know it's not because they don't want the aircraft to get lost in space, because the max. alt. of the Airbus A350 is at FL420, which is not even close to space. Anyone ave an answer?

Have you seen this question and this one?

@Pondlife: Yes I have, but that's about the legal status of planes flying above the max. altitude. Not what physically happens to the aircraft and the poeple inside of it.

There is a variety of reasons for a maximum altitude:

Most common is insufficient engine performance to climb any higher. Then your question never arises. The aircraft simply cannot go higher.
For more powerful aircraft the partial pressure of the oxygen supply in an unpressurized cabin would drop below the value to keep the pilot(s) alert. For GA aircraft without supplemental oxygen this would be 15,000 ft (short term) or 12,500 ft (long term).
For many airliners it is the pressure limit of the cabin which is certified for a maximum differential to outside pressure. If the airplane climbs higher, the internal pressure must be reduced to less than what is deemed the acceptable minimum for the passengers.
For special subsonic high altitude aircraft with sufficient engine power the limit is the coffin corner when minimum speed and maximum Mach fall together. When that height is exceeded, the aircraft stalls and exceeds its Mach limit at the same time. The result is a shallow dive which can only be recovered a few thousand feet lower.
In some cases the limit is given by minimum combustion chamber pressure inside the engine to keep the flame burning. There are cases when aircraft climbed too high, suffered a flame-out and could not restart.
With supersonic aircraft the limit might be set by maximum flight Mach number, when higher speed means stagnation heat exceeds structural limits. The high Mach number is needed to sustain minimum dynamic pressure to produce enough lift.

thanks for the answer.

The maximum altitude specifications that an airplane has are based on the practical limits on staying airborne as the plane climbs higher and higher. For ordinary planes like airliners and private planes, those limits do not mean something "breaks" if you fly too high, the way it would if you flew too fast in a dive for example. It means that if you try to fly higher than a certain altitude, the engine's performance is degraded enough that the plane can't climb with its full rated payload any higher than that.

thanks for being the first one to answer my question.

A typical airplane needs two things to stay airborne: sufficient airspeed (specifically, indicated airspeed, which is directly related to dynamic pressure which is what makes wings generate lift), and to stay subsonic. The latter criterion is due to the design of the airplane; supersonic airplanes do not have this limit and can typically fly much higher.
As you go higher, for a given true airspeed, the indicated airspeed decreases. To get enough dynamic pressure on your wings, you need to increase speed. However, the speed of sound decreases even faster with altitude, which means at some point you'll need to go near supersonic to get enough airspeed to keep enough dynamic pressure on your wings to keep aloft. Go faster and you'll go supersonic and plummet out of the sky; go slower and you'll stall and plummet out of the sky. This point is called the coffin corner, and typically, you want to stay away from it (even though there's much sky underneath you to plummet into).

thanks for the answer.


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How good is an online ground school training?
I'm a software engineer and an aviation enthusiastic person.
I'd like to get a PPL while continuing my current job and I found there are lots of online ground school trainings are available.
My question is how good are these training materials? and is it a good idea to do the ground school classes online without going to a school?

Welcome to AviationStackExchange. The question that you asked is definitely not a question that solicits fact based answers. Something that is usually frowned upon on this question and answer forum. Hence the down-votes and close-votes from others. My answer (as well as any answer to this question) will be very biased and opinion based.
In answering this question, I would like to separate your time spent on ground school into two categories. Those are:

Ground school in preparation for the knowledge exam / written test.
Ground school in conjunction with flight training.

If you are preparing for the written test, online ground schools are a great supplement to the FAA provided materials. All of the knowledge exam questions are taken directly from or have a reference to either an FAA official Handbook, Manual, Advisory Circular or other publication. Each publication and subsequently each question is written by government officials with the review of government attorneys. They can be very dry and sometimes hard to understand. Most of these publications can be downloaded for free from the FAA website.
Many online ground schools will teach these subjects in a way that is more easily understood and palatable than studying on your own. Plus, you can immerse yourself in aviation by replaying the lessons while doing other activities like driving or cleaning the house. Studying the lesson with total concentration, then replaying the lesson throughout the day will help the material sink in. Many also have free tutorial YouTube videos and podcasts to help with that immersion. My recommendations for that are:

King School
M0A or MZeroA
Fly8MA
The Finer Points
Aviation 101
Rod Machado
Aviation Training Network
Bold Method

Ground school training in conjunction with flight training is another story. You will sometimes hear the old aviation trope/saying, “The cockpit is a horrible classroom.” It is very hard to learn the academics of flight in the cockpit. It is even harder to absorb the material while dealing with the stress of flying. Investing the time and considerable amount of money to learn face-to-face with an actual instructor in real time is invaluable. It is also more effective and cheaper than trying to do the same while paying for a running aircraft engine.
Each flight should begin and end with a ground school lesson. Even a short one. At the very least, a detailed briefing and a debriefing should be done. You will find it more advantageous to do those sitting comfortably in a building rather than in a cramped cockpit on a noisy ramp. Some subjects like weather, physiology, Aeronautical Decision Making, flight planning, etc. are best taught in a classroom setting on the ground. Some ground school lesson will be so intensive that they will need to be done on separate days from your flights. Don’t shy away from nor neglect to take advantage of this investment. It will save you time and money in the long run. Plus, you can log that time in your logbook as proof that some type of ground instruction was done for certifications, ratings, endorsements, flight reviews, etc.
My recommendation would be to first get your medical exam done if it is required for your ultimate desired pilot certificate goal. Next, find and interview many different flight schools and CFIs to find one most conducive to your schedule and personality. Go for a Discovery Flight to see if this activity and instructor are the right one for you. Then, ask the instructor with which online ground schools they are most familiar. Use that ground school to prepare for the knowledge exam. That way you can discuss with them and ask questions regarding specifics from the lessons. Procure sample knowledge exams to study in detail. The FAA are notorious for the way they structure their questions and answers. I would recommend exam study programs like:

Flight Test 5 from Sheppard Air, Inc.
Study Buddy from Sporty’s Pilot Shop
Prepware from ASA (Aviation Supplies & Academics, Inc.)

Then, take and pass all of the knowledge exams up to the level of pilot certificate you would like to achieve before starting the rest of your flight training. It is totally possible to do well on these exams without ever having set foot in an airplane. And, it will prepare you well to be a student pilot before your flight training. It’s also a weight off of your shoulders to have one less thing to worry about during your training.
*Disclaimer: The views, recommendations and opinions of this poster do not reflect those of AviationStackExchange.com.


text:
Could any complications arise if the two FMCs are using different OPS programmes, i.e U10.7 and U10.8A?
B737-300
FMCs ARE OF SMITH INDUSTRIES 2907a4/10-6225-003

You should explicit technical terms you use. Although the community here is expert in aviation, this field is huge and no one can be expert in all subfields. Thus you may at least provide links to explain all references.

It wouldn't work. The two FMC's need to communicate & stay in sync; that process requires the same OFP in both.


text:
How can I get a license to fly an F14?
Say I have $x million to buy an F-14 Tomcat and am physically fit (I think) to be able to fly it. Is there a short pilot training course I can attend to get licensed to fly it?

Does this answer your question? What licenses/endorsements or type ratings do you need to fly a fighter jet?

Define "short". Current US Navy training to fly their strike aircraft takes about two years (Source)

Getting licensed to fly an F-14 is one thing; renting an F-14 to fly is another.

Anybody with the resources and connections to buy an F-14 could probably find a former Navy CFI to get them up to speed.  Otherwise no, there is no existing course you can simply attend.  There just isn't enough demand to run such a business.


text:
Studying bird flight and insect flight: Would studying aerodynamics of airplanes be useful?
Would studying the aerodynamics of airplanes be useful, if one wants to study bird flight and insect flight?
For instance, would studying a gliding paper plane and a gliding unpowered airplane be fundamentally the same?
I've read that aerodynamic laws for airplanes are different from aerodynamic laws governing bird and insect flight, but I forgot where I read this from -- possibly from an academic journal paper, if I remember correctly.

I'm not sure this is really on topic here. you might get better answers from someone who actually studies bird and insect flight. Take a look at [biology.se]

related: Could the principle of owls' silent flight be used for stealth aircraft?

The biggest factor, besides the basic insect ornithopter wing mechanics, is air viscosity effects, or Reynolds Number.  To insects, the air is extremely "thick" with extremely low Reynolds Numbers, and the optimal airfoils are single surface membranes - insect wings. As Rn gets higher, the desired airfoils get thicker (bird wings) and thicker until you are in very high speed flight and the wings have to start getting thinner again.
Anyway, for a basic overview of all of this, I can't recommend the book Miracle of Flight by Stephen Dalton highly enough.  It's a book for the layperson that illustrates flight pretty much up the Reynolds Number range, starting with insects, then birds, then the man-made stuff.

The book is written for a young layperson adult I'd say.  Definitely not an engineering dissertation; however, the photography is really beautiful, his writing is clear and concise and he's researched it really well, so it's a fascinating read, laid out in a way I've never seen before or since. Presenting flight through the Rn range is just brilliant. So at a pure technical level, it certainly won't be what you're looking for, but it's so well presented it's an enjoyable read, if only as an introduction to the topic.  You may discover insights you never thought of that lead you somewhere else.

With birds, it likely depends on the size of the bird and their manner of flight.  At one end of the scale, to the casual observer hummingbirds are pretty much identical to hummingbird moths.  At the other, the larger soaring birds are quite similar to gliders.

If you are interested in flight strategies of soaring birds-- why they would elect to fly at a certain airspeed when flying in a headwind versus a tailwind-- why they would adopt a different wing shape when gliding at a high airspeed than when circling slowly-- then you can certainly learn a tremendous amount from the literature on human soaring flight.  The basic principles are the same.  See also the book "Flight Strategies of Migrating Hawks"-- a little dated now, but still lots of good stuff in there.  Doesn't pertain too much to flapping flight though, for the most part.


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Why do airlines not paint the wings on their livery? Why?
Does anyone know why airlines do not paint the wings on their aircraft? What is the reason why?

Some do. However, paint is expensive and heavy. You may find that getting approval to paint a lift producing surface is more difficult to justify to the regulatory authorities. And also, why paint somewhere you can't see very well when it adds weight and therefore burn more fuel?


text:
Why do the landing gears of Boeing 767 and certain Airbus aircraft tilt down?

I need help and some explanation on why are they sometimes tilted down instead of up.

Hopefully the linked duplicate (being the opposite of this question) answers this question.


text:
What are the Requirements for the implimentation of Low Visibility Procedures at an airport?
So I was wondering if the RVR at an airport is reported above CAT I conditions for eg 1400 Meters etc but the Visibility is 50 to 100 Meters then would the pilots carry out non instrument approach or would the approach would be carry out as per Low Visibility conditions ?
And what are the conditions for an airport to impliment Low Visibility conditions ?

https://www.skybrary.aero/index.php/Low_Visibility_Procedures_(LVP)

You probably need to correct the use of the term "non instrument approach".

If prevailing visibility is that different from the RVR at a runway, something is probably wildly off (i.e. one of the sensors is reporting inaccurately). Maybe an incredibly thick fog bank at the PV sensor but not at the runway, but that's rare. Almost always, RVR readouts are within +/- 1/4 SM (converting units, of course) of the reported visibility.

It is the RVR values that are used to determine the applicable minima.
The switch from normal to LVP Ops varies from airport to airport but the changeover process typically commences when the dropping vis approaches 1000mtr (say roughly 1200m) and is dropping so that the ILS Cat2/Cat3 LVP can be offered as the vis/rvr falls to below Cat1 requirements. More detailed info can be found in the country AIP and the same info would be spelt out in the Jepp or equivalent.
The Low Visibility info is specified in Sec 1.3 in the 10-1P series pages of the Jeppessen Terminal charts, ie "Airport Briefing" pages.


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Is there any latest List of CAT IIIB airports in the world?
Recently I had my very first CATIIIB experience at my own homebase (VILK) and since then I've been keen to know more about this most advanced landing system we have in the aviation as of now and given its such a costly instrument and its maintenance is no easy feat I was wondering how many airports around the world may have this technology..does anyone have any data ?
Thank you

Related: Is there a list of airports with Category 3 ILS systems?

Thanks for that Mr Bianfable..but this list seems to be outdated now.

@Avgeeker That list was from Dec 2019, and it’s unlikely to have changed much in 15 months.

It doesn't have BLR The latest airport to be equipped with CATIIIB ILS and I was looking for the list of CATIIIB airports specifically I know the number of CATIIIB airports is far less than CATIIIA like Canada doesn't have a single CATIIIB runway ..so I'm looking to make a list of only CATIIIB airports for my research.

Prishtina Internationell Airport have Cat III B


text:
What other materials apart from conventional nickel and steel superalloys can be used as a first stage turbine blade material?
Steel and nickel chromium alloys have been prominent and most used as turbine blade material although there are other materials such as tantalum carbides, tungsten and many more which I have no knowledge of. But these materials are not being used or even planned on being used in the near future why is that?
Do the cons of these materials beat the pros of the ni-cr superalloys?

A close match as a question: https://aviation.stackexchange.com/q/14454/42636

Which material would be able to replace ni-cr alloy in future?

That would actually be a question for material sciencentists, not aviators

I doubt anyone in the trade would call any first stage turbine material a "steel" although many contain some iron. Vitallium was one of the first alloys  Cr and Co ( no iron). Carbides are not used as such although there is a myriad of intermetallic particles , some  are carbides, that form in the alloys as they are heat-treated. I expect there will always be alloy development but many advances in blade and vanes are in ; surface coatings , axial cooling passages, orientated crystals / grains , single crystals , and things I never heard of. Apologies in advance to anyone knowledgeable in the field. Wikipedia has a fair introductory entry for gas turbine blade materials . One correction , they mention steam turbines which use dramatically different materials ; primarily 13 Cr martensitic stainless , I expect steam may also use low alloy steels but we only saw the failed 13 Cr blades.

Tungsten for one has two major flaws when compared to other candidates listed.
The first one is actually told by the name of the material: tungsten -> tung sten = "heavy rock" in swedish. It is one of the heaviest elements having about the same density as gold. Even if it was a match in other properties for superalloys (which it is not), the weight of components made from it would be a serious problem. Heavy rotating masses are not desirable.
The second problem is the difficulty of the "workability" of tungsten. Very high melting point makes casting difficult, and it is also a very hard material to machine.

Turbine blades must withstand high tensile stresses at their blade roots and be resistant to cyclic stresses from fluctuating fluid dynamic loads- all of this at very high temperatures, and over great numbers of thermal cycles. This argues in favor of metallic blades and against ceramics in this application, because they perform better against fracture in tension and fatigue crack propagation.


text:
What types of airfoils do fighters usually use for their horizontal and vertical stabilizer?
I'm looking for different types of airfoils that fighters have used for their tail, such as the F18 or F22. Or in general what airfoils do fighters typically have. For instance do they want low t/c, do they want to be symmetrical (regarding the horizontal tail), do they want to be cambered, etc.

The best resource for airfoil usage is the UIUC Incomplete Guide.
Unfortunately, empennage airfoils are rarely revealed and you won't find many in the guide.
Fifth gen fighters with subsonic leading edges tend to use NACA 65A or 64A for their wings.  Fighters with supersonic leading edges use a bi-convex or flattened hexagon kind of airfoil.
Sixth gen fighters likely use similar airfoils for their wings, but may use airfoils tailored for low observables, or perhaps custom airfoils that are similar in design, but with the prevalence of modern tools there is little barrier against designing a custom airfoil.
Vertical tails are almost always symmetrical -- horizontal tail airfoils are frequently symmetrical.  While there could be some benefit to a cambered horizontal tail (probably upside-down), it is likely not worth the trouble.  Many high performance aircraft use an all-moving tail.
High speed aero wants low t/c.  Structures wants high t/c.

Thank you for your detailed response!


text:
What is the taper ratio approximately for the horizontal and vertical stabilizer of a fighter jet and what determines this taper ratio?
I'm looking for historical taper ratios for the tails of different fighter aircrafts, or just a general fighter aircraft. Also why do fighter aircrafts want their taper ratio at a certain value. Cheers!

According to Aircraft Design by Raymer, taper ratios for the horizontal and vertical tail for fighters are generally 0.2-0.4 for both.


text:
How big are the horizontal stabilizers for fighter jets generally? (ie. 25% of the wing area)
Anyone know the typical area for horizontal tails for fighter jets like the F-18.
Cheers

In pre-design phase the so-called volume method is used.
Tail counteracts the moments created by the wing and it does it creating an aerodynamic moment proportional to its area and its leverarm (area times leverarm is a volume, that's why the name):

vertical tail surface is proportional to $S_v=\frac{c_vbS}{L_v}$
horizontal tail surface is proportional to $S_h=\frac{c_h \cdot mac\cdot S}{L_h}$

where $c_v$ and $c_h$ are historical coefficients, $L_v$ and $L_h$ are vertical and horizontal tail's leverarms respectively and $mac$ is the wing mean aerodynamic chord.
According to Raymer¹, $c_v$ is between 0.07 and 0.12 for jetfighters, $c_h$ is 0.4 and leverarms for rear-mounted engine aircrafts are both some 0.45 to 0.5 of fuselage's length.
Just as an example let's do the math for the vertical stabiliser of the F-18 using intermediate values of the just given coefficients:
$S_v=\frac{0.095\cdot11.4\cdot37.15}{17.1\cdot0.475}=4.95m²$
F-18's actual vertical stabiliser surface is 4.85m², a very good result.

¹ Daniel P. Raymer, Aircraft Design: A Conceptual Approach, AIAA Inc.

Thank you for the response. I am using Raymer's method to size the tail for a concept design and wanted something to compare my values with!


text:
Do manufactures list the maximum speed of an aircraft based on its theoretical maximum speed or dive speed?
I know that the absolute maximum velocity of an aircraft is based on the thrust available. However the aircraft is usually limited beforehand by other speed limits such as dive speed and Mach Never Exceed velocity.
So when researching the maximum velocity of an aircraft, is the max speed listed the absolute max speed or the dive speed as that would make the most sense since its the structural limit. The example I am specifically thinking of is the C-130J Hercules which according to this brochure (linked below) from Lockheed, is around 365 kts.
https://www.lockheedmartin.com/content/dam/lockheed-martin/aero/documents/C-130J/C-130Brochure_NewPurchase_May2020_Web.pdf

Is that maximum dive speed with the wings still attached or without? I'd think the latter would be somewhat higher...

"is the max speed listed..." listed where?   In the listing for Vne?  Or in the promotional brochures regarding the performance of the aircraft, which would presumably be in level flight?

I should have been more clear, I understand that the maximum speed on the brochure is for steady level flight. I was confused as to which speed limit in level flight however. I think I understand now though, thank you.

The maximum cruise speed from the brochure is a True Airspeed value and is based on the highest TAS achieved with the engines operating at the maximum cruise power rating specified by the engine manufacturer, based on some altitude and load condition that is probably buried somewhere in the fine print (we could probably assume the 365 kt value is at max cruise power, max gross weight minus departure and climb fuel, at service ceiling, in ISA conditions).
Note that limitation speeds like Vmo or Vd are based on indicated/calibrated airspeed (CAS, what the pilot sees as a speed on the instruments) and are a function of the dynamic pressure conditions acting on the airframe, regardless of the actual velocity through the air mass (True Airspeed).  They are generally a lower value, as TAS starts to exceed IAS as soon as you go above sea level, the difference getting bigger and bigger the higher you go.
So at the 28000 ft service ceiling, the indicated airspeed will only be about 230 kt when the Herc is cruising at a TAS of 365 kt on a standard day with a -40F temperature outside.
The speed limitations will be a margin above the indicated/calibrated cruise airspeed value;  if, say, the Vd was 290kt (I'm just picking a number at random), at 28000 ft in standard conditions, that's a TAS of 450, but that's irrelevant for the purpose of the speed limitation because we're only interested in the IAS value insofar as that's what the pilot sees, and it's what represents the actual dynamic pressure effects of that speed for structural purposes (although where the limit is for flutter considerations, that are a function of TAS, the flutter TAS limit is converted to IAS in the Air Data Computer for the pilot to use).

The never exceed speed may be a true airspeed limit since flutter depends on true airspeed, though it will still be given as indicated airspeed limit, tabulated dependent on altitude.

Modified my post thanks.  I was only thinking of structural effects.

Yeah I took out No.  Thanks

Thank you for your response!

The 365 knots listed in that brochure is (as noted in the text) the maximum cruise speed. That is the maximum speed at which the aircraft can be expected to operate under normal circumstances in level flight. It is not related to the Never Exceed speed, which depends on the structural limits of the aircraft (as well as air pressure and atmospheric conditions etc.). The maximum cruise speed is interesting for planning purposes, as it gives you an indication of how far you can expect the aircraft to go in a certain time window. It is not, however, very interesting to pilots, who will operate the aircraft with regards to the actual structural limits.

"It is not, however, very interesting to pilots, who will operate the aircraft with regards to the actual structural limits." Not if they want to keep their jobs. I imagine that the airline might get rather annoyed at them burning a bunch of extra fuel to get to their destinations marginally faster!

@nick012000 I'm not saying pilots will try to fly at Vne, but rather that the factors they use to determine which speed to fly are different from the ones used to calculate the max cruise speed printed in a sales brochure

Thank you for your response!

@expeditedescent I'm not saying that they'll fly at Vne either - I'm saying that they'll fly at the most fuel-efficient speed, which is likely the cruise speed listed in the manual.

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